1. Field of the Invention
This invention relates to a sparkplug voltage detecting probe device which detects a sparkplug voltage waveform applied to a spark plug in an internal combustion engine.
2. Description of Prior Art
With the demand of purifying emission gas and enhancing fuel efficiency of internal combustion engine, it has been necessary to detect firing conditions in each cylinder of the internal combustion engine so as to protect the internal combustion engine against any type of misfire. In order to detect the firing condition in each of the cylinders, the applicant of the present patent application discloses a misfire detecting device in patent application No. 3-80107. In this device, a sensor is provided to form a static capacity between the sensor and a secondary circuit of an ignition coil, and a sparkplug voltage divider circuit is provided to divide the sparkplug voltage by the static capacity and a capacitor connected between the sensor and the ground. In parallel with the capacitor, a resistor is connected to form a RC path. The step-up level of the sparkplug voltage is detected after a spark plug terminates the spark action. The device is made on the fact that the step-up level is enhanced more when a misfire occurs than when the normal combustion occurs.
In this device, it is, however, necessary to determine a time constant of the RC path to be approximately 10 milliseconds so as to detect a sparkplug voltage waveform within an ignition timing of all the rpm region from low rpm to high rpm of an internal combustion engine.
When it takes long time for the step-up sparkplug voltage to be released, the subsequent spark action has been initiated before the capacitor had been completely discharged to raise a zero point level (reference voltage level) more than the ground voltage level. For this reason, the average level of the sparkplug voltage becomes relatively high to raise the zero point level so as to make the sparkplug voltage waveform sink under the zero point level. This is particularly true when the misfire often occurs, otherwise the spark action includes ineffective spark discharge which appears in a stroke of exhaustion as with the case in which a distributorless igniter (DLI) is incorporated into a sparkplug voltage detecting probe.
From the reason that the burning conditions in a cylinder are detected by measuring the sparkplug voltage level exceeding the zero point level, the detection precision the burning conditions deteriorates when the step-up level of the sparkplug voltage after the end of the spark action is not high with the engine running at low rpm. Especially in the sparkplug voltage detecting probe into which DLI is incorporated, the variation of the sparkplug voltage waveform caused from the ineffective spark discharge is small so as to worsen the detection precision.
Therefore, it is an object of the invention to provide a sparkplug voltage detecting probe device for use in internal combustion engine which is capable of preventing the zero point level of the reference voltage from inadvertently rising, and precisely detecting a characteristic of the sparkplug voltage waveform in all the rpm region from low rpm to high rpm of an internal combustion engine.